This invention relates to dental preparations and, more particularly, to powdered alginate compositions which are adapted to be admixed with water to provide orally settable, dental impression material.
Alginate compositions have long been used in dentistry as impression material for making impressions in areas in which partial dentures are to be constructed, for fabricating study models in orthodontic treatment, for making primary impressions in endentulous mouths, and as corrective materials in secondary impressions of all types.
As distinguished from agar-based thermally reversible hydrocolloids which gel by lowering the temperature of the heated and fluidized material, alginate compositions gel by means of a chemical reaction. After the alginate gel is formed, it cannot be converted to a fluid condition or sol by physical means and, thus, the alginates are known as irreversible hydrocolloids. The use of alginates in dental therapeutics is reviewed in the text entitled Elements of Dental Materials, by Ralph Phillips at Chapter 9.
Alginate compositions designed for use in dental therapeutics are typically formulated as powders which are adapted to be mixed with water to form a viscous sol. The sol is carried into the mouth in a perforated impression tray where it forms an elastic gel through a series of chemical reactions. Following formation of the gel, the impression is removed from the mouth for use in the construction of dental forms.
The basic ingredients of an illustrative powdered alginate comprise potassium alginate, calcium sulfate dihydrate, trisodium phosphate and diatomaceous earth, to which there may be added special purpose ingredients such as magnesium oxide and potassium titanium flouride as well as ingredients for color, flavor and preservation.
The significant ingredient in the powdered alginate composition is soluble potassium alginate which is derived from sea kelp. When the powdered alginate composition is mixed with water, the soluble alginate reacts with the calcium sulfate to produce the gel structure of an insoluble calcium alginate. Since this reaction must take place and go to completion in the mouth, it must be delayed until the aqueous composition is placed in the impression tray and carried to the mouth. In order to effect this delay and provide adequate working time, a reaction rate retarder such as trisodium phosphate is incorporated into the composition. The suggested mechanism for the effectiveness of the reaction rate retarder is that the calcium sulfate will react first with the trisodium phosphate before reacting with the soluble alginate and that as long as any trisodium phosphate is present, the gelling reaction between the soluble alginate and the calcium sulfate will be prevented. A filler such as diatomaceous earth is also incorporated into the formulation to increase the strength and stiffness of the gel and to provide a firm surface that is not tacky. The final structure of the gel is characterized as a brush-heat network of fibrils of calcium alginate which holds the excess water, filler and other ingredients.
The following prior art patents illustrate the state of the art with respect to settable alginate compositions:
U.S. Pat. No. 2,657,971 (Lochridge, 1953) discloses the use of tetrasodium salt of ethylene diamine tetra acetic acid as a reaction rate retarder in a powdered alginate composition containing, for example, potassium alginate, calcium sulfate, diatomaceous earth and sodium fluosilicate.
U.K. Pat. No. 936,091 (Lonsdale, 1963) discloses that dimensionally stable impressions can be obtained by the use of sodium flouride and potassium fluorortitanate in a powdered alginate composition containing sodium alginate, calcium sulfate dihydrate, sodium tripolyphosphate, diatomaceous earth and a blend of light and ultra-light magnesium oxide.
U.K. Pat. No. 951,547 (Nordin, 1964) discloses that dimensionally stable impressions can be obtained by the addition of powdered aluminum to a powdered alginate composition containing sodium alginate, calcium sulfate, sodium flouride, zinc oxide, magnesium oxide, sodium tripolyphosphate, and diatomaceous earth as well as lead silicate.
U.S. Pat. No. 3,268,348 (Morrell, 1966) and U.K. Pat. No. 1,159,471 (Morrell, 1969) disclose, respectively, the use of barium fluoride and lithium fluoride in powdered alginate compositions containing potassium alginate, calcium sulphate dihydrate, tetrasodium pyrophosphate, magnesium oxide and diatomaceous earth whereby impressions made therefrom do not require any treatment in an aqueous fixing bath containing an inorganic salt and that models or casts produced from such impressions possess hard smooth surfaces, which are free from chalkiness and dusting in handling during use of the same or in storage.
U.K. Pat. No. 1,365,024 (Marrickville Holdings Ltd., 1974) discloses that dental impression material having improved flexibility and strength can be prepared from powdered alginate compositions containing potassium alginate, tetrasodium, pyrophosphate, sodium silicofluoride, sodium metasilicate, sodium fluoride, zinc fluoride and synthetic calcium sulfate comprising a blend of calcium sulfate hemihydrate and calcium sulfate dihydrate.
U.S. Pat. No. 4,381,947 (Pellico, 1983) discloses an interactable, two-component, paste system for preparing alginate impression material wherein one component contains an alkali metal alginate in an aqueous paste and the other component contains a divalent metal salt such as calcium sulfate and a reaction rate retarder such as tetrasodium pyrophosphate in a fluid plasticizer that is substantially free of unbound water.
The powdered alginate compositions of the prior art, upon admixing with water to form dental impression material, tend to appear grainy and form lumps. This phenomena is primarily associated with the alkali metal alginate ingredient. Accordingly, it would be advantageous to provide powdered alginate compositions which can be smoothly admixed with water without appearing grainy or forming lumps.